Nansen was the son of Baldur Fridtjof Nansen, a lawyer, and Adelaide Johanne Thekla Isidore Wedel-Jarlsberg, a niece of Herman Wedel-Jarlsberg. Both his parents had been married before and each had children from their previous marriages. These half brothers and sisters, who were many years older, looked after Fridtjof and his younger brother, Alexander, during their childhood. The older half brothers especially played a major part in laying the foundation for the love of and affinity to nature which Fridtjof developed, and which followed him throughout his life.
The Nansen family was well-disciplined, they admired sports, especially skiing - it was cultivated by Adelaide Nansen. Fridtjof was taught to ski from the age of two. The children did not have purchased toys; Fridtjof made himself a bow and arrow, fishing rods, and windmills on the stream. As a teenager, he and his brother Alexander were allowed to spend a lot of time in the forest, Nansen even compared himself to Robinson.